A nuclease has been isolated and largely purified from a marine Pseudomonas strain which breaks a DNA chain everywhere a single-strand scission occurs, attacks supercoiled duplex circular DNA to yield linear duplex molecules, extensively degrades single-stranded DNA, and, upon prolonged incubation, shortens linear duplex molecules progressively without the introduction of single-strand breaks into the duplex. Closed circular DNA's with low numbers of superhelical turns are not attacked. Preliminary work has shown an RNase activity to be present as well. All but the RNase activity (not yet tested) are properties of a single protein band in gel electrophoresis experiments. Supercoiled closed circular molecules are not attacked at a single unique site in the genome. All activities persist at ionic strengths in excess of 4 M or in the presence of 5% sodium dodecyl sulfate. Calcium ion is absolutely required for activity; irreversible inactivation is observed upon removal of this ion. Magnesium ion is also necessary for full activity; this effect is reversible. The enzyme is very active at pH near 8; the optimum pH has not yet been determined. Work is proceeding toward obtaining even purer samples and ascertaining the physical properties of the nuclease, as well as characterizing the terminal products of nucleic acid degradation. Possible useful aspects of this highly stable nuclease, such as removing all non-duplex structure from heteroduplex nucleic acids, are to be investigated. Preliminary evidence has been obtained that the nuclease will cleave DNA at the site of a single mismatched base "pair"; this aspect is being further pursued. BIBLIOGRAPHIC REFERENCE: Gray, H.B., Ostrander, D.A., Hodnett, J.L., Legerski, R.J., and Robberson, D.L., "Extracellular Nucleases of pseudomonas BAL 31. I. Characterization of Single Strand-Specific Deoxyriboendonuclease and Double-Strand Deoxyriboexonuclease Activities", Nucleic Acids Res. 2, 1459-1492 (1975).